The invention relates to a process for making propylene polymers or copolymers in propylene medium.
Several processes for polymerizing alpha-olefins, for example propylene, are known. Such processes where Ziegler-Natta catalysts are employed, are for example slurry polymerization carried out in a solvent such as n-hexane, bulk or slurry polymerization carried out in a liquefied alpha-olefin monomer such as propylene and gas phase polymerization carried out in a gaseous monomer such as gaseous propylene. Further, combinations of these processes are also known such as slurry polymerization followed by gas polymerization.
Gas phase processes are advantageous in that recovery and reuse of inert hydrocarbon or monomer is more simple than in slurry processes. The cost for equipment for monomer recovery and reuse is small compared to slurry processes. One disadvantage of the gas phase processes is that the monomer inside the reactor is in vapor phase and therefor the monomer concentration is relatively low compared that of slurry processes. This results in a lower reaction rate. In order to increase the polymer yield per unit weight of catalyst, it is necessary to extend the residence time in the reactor by increasing the volume of the reactor.
In a book Y. V. Kissin, Kinetics of Polyolefin Polymerization with Heterogenous Ziegler-Natta Catalyst (1981), p. 10,11,70,71,125 the influence of temperature in propylene polymerization with TiCl3-based Z-N catalysts has been discussed. The active centers of catalysts have been shown to be stable up to 80xc2x0 C. In a polymerization process carried out at relatively high temperatures, eg. 70-80xc2x0 C. and high monomer concentration, the stage at which the rate of chain initiation and chain termination are equal, is reached early.
The overall polymer yield of such catalysts is in general low and very costly ash removal is necessary in the process.
According to EP0417995 a special catalyst for propylene polymerization at very high temperatures of 150-300xc2x0 C. is disclosed. The catalyst has a typical structure which is possible with a claimed organoaluminium component and a silicon compound. However this process is not practical because the proposed polymerization temperatures are higher than the melting temperature of polypropylene.
Sergeev et al. (Macromol. Chem., 185, (1984), 2377-2385) have observed with TiCl4/EB-AlEt3/EB catalysts a slight increase of isotactic index when passing from 20xc2x0 C. to 60xc2x0 C. and a rapid decline above 70xc2x0 C. Further Spitz and Guoyt (Macromol. Chem., 190 (1989), 707-716) reported for MgCl2/TiCl4 catalyst that the number of active centers remains constant within the range of 50-70xc2x0 C. Above 80xc2x0 C. the activity decreases and the catalyst is deactivated.
In many patent applications it is mentioned that higher temperatures, such as up to 100xc2x0 C., could be used. However in such publications, for example EP0438068 and EP0412750, only lower temperatures of 70-80xc2x0 C. are presented in the examples. Therefore, according to prior art only lower temperatures of up to 80xc2x0 C. has been used.
From U.S. Pat. No. 5,093,415 it is known a high temperature (over 100xc2x0 C.) process employing a special catalyst containing magnesium, titanium, halide and carboxylic acid ester containing two coplanar ester groups attached to adjacent carbon atoms. However this is a gas phase process and comparative examples at lower temperatures show activity decrease above 80xc2x0 C.
Finnish patent application 954814 concerns a process for polymerizing propylene in at least one slurry reactor, where the temperature and the pressure are above the supercritical temperature and pressure of the reaction mixture. One of the main advantages of operating under supercritical conditions is that great amounts of hydrogen can be freely added to the slurry reactor because hydrogen readily dissolves into the supercritical fluid.
The present invention concerns a multistage process for homo or copolymerizing propylene, wherein propylene is polymerized in the presence of a catalyst system comprising a procatalyst component and a cocatalyst component, said procatalyst component comprising magnesium, titanium and at least one internal donor compound, at an elevated temperature in a reaction medium, in which a major part is formed by propylene. The present invention is characterized in that the polymerization is carried in at least one slurry reactor in the presence of liquid propylene at a polymerization temperature between 80-91xc2x0 C. and by using a catalyst system where said internal donor compound is slightly soluble, the amount of said slightly soluble donor compound in the catalyst system being at least 1 w-%. This kind of catalyst system produces within said temperature range a high productivity and essentially constant isotacticity within wide melt index range.
According to another embodiment of the invention the catalyst system can include at least two internal donor compounds, of which one is slightly soluble internal donor compound and another internal one donor compound is easily soluble, and the amount of said slightly soluble donor compound in the procatalyst is at least 1 w-%.
According to the invention it has been found that by using in propylene polymerization a catalyst system having at least one internal donor compound which is slightly soluble in eluting agents and by using this donor compound in a certain amount a highly stereospecific catalyst system is obtained which give certain performance between temperature range of 80-91xc2x0 C. First, the catalyst system gives a high productivity and secondly, the catalyst system gives relatively high isotacticity index, which remains essentially constant although polymers have varying melt index. With ordinary Ziegler-Natta catalysts the isotacticity index is at a lower level and drops when the melt index increases.
Examples of the catalyst systems, which are usable according to the invention, among others, are generally disclosed for example in Finnish patents FI86866, FI96615, FI88047, FI88048 and Finnish patent application FI963707. These catalysts have been presented for use only in relatively low temperatures.
According to this invention a suitable catalyst system comprises a procatalyst composition prepared from magnesium dichloride, titanium compound and at least one internal donor compound having a slight solubility in hydrocarbons or compounds used as cocatalyst, and a conventional cocatalyst compound. According to one embodiment of the invention the procatalyst composition is obtained by applying transesterification method, which is generally disclosed for example in Finnish patent 88048. The transesterification reaction is carried out at an elevated temperature between a lower alcohol and a phthalic acid ester, whereby the ester groups from lower alcohol and phthalic acid change their place.
MgCl2 can be used as such or it can be combined with silica, e.g. by absorbing the silica with a solution or slurry containing MgCl2. The lower alcohol used can be preferably methanol or ethanol, particularly ethanol.
The titanium compound used in the preparation of the procatalyst is preferably an organic or inorganic titanium compound, which is at the oxidation state of 3 or 4. Also other transition metal compounds, such as vanadium, zirconium, chromium, molybdenum and tungsten compounds can be mixed with the titanium compound. The titanium compound usually is halide or oxyhalide, an organic metal halide, or a purely metal organic compound, in which only organic ligands have been attached to the transition metal. Particularly preferable are the titanium halides, especially TiCl4. Preferably the titanation is carried out in at least two steps.
The transesterification can be carried out e.g. by selecting a phthalic acid esterxe2x80x94a lower alcohol pair, which spontaneously or by the aid of a catalyst, which does not damage the procatalyst composition, transesterifies the catalyst at an elevated temperatures. It is preferable to carry out the transesterification at a temperature, which is between 110-150xc2x0 C., preferably between 115-140xc2x0 C.
The alkoxy group of the phthalic acid ester used comprises at least five carbon atoms, preferably at least 8 carbon atoms. Thus, as the ester can be used for example propylhexyl phthalate, dioctyl phthalate, di-nonyl phthalate, di-isodecyl phthalate, di-undecyl phthalate, di-tridecyl phthalate or di-tetradecyl phthalate.
According to the invention the slightly soluble internal donor compound is a C1-C6 alkyl ester of organic carboxyl acid, preferably C1-C6 alkyl ester of organic dicarboxyl acid and most preferably diethyl phthalate. It is very difficult to elute diethyl phthalate from solid catalyst with solvents or eluents. The reason may be that small alkyl groups of the phthalate, i.e. ethyl groups, do not solvate easily with hydrocarbon or hydrocarbon containing solvents. Thus the slightly soluble internal donor is not removed when the procatalyst compound is treated with eluents or cocatalyst compositions and therefore the stereospecifity remains also at high polymerization temperatures.
Eluents are organic and metalloorganic compounds such as compounds of Group 1, 2 or 3 metals containing C1-C10 alkyls. Preferably, the eluent is a metal compound containing C1-C10 alkyls, which are used also as a cocatalyst. The preferable eluent is tri-C1-C6-alkylaluminium, more preferably tri-C1-C4-alkylaluminium, most preferably triethylaluminium.
According to one embodiment of the invention propylene is polymerized in the presence of a catalyst system comprising a procatalyst component and a cocatalyst component, said procatalyst component comprising magnesium, titanium and at least two internal donor compounds, at an elevated temperature in a reaction medium, in which a major part is formed by propylene, whereby the polymerization is carried in at least one slurry reactor in the presence of liquid propylene at a polymerization temperature between 80-91xc2x0 C. and by using a catalyst system in which one of said internal donor compounds is slightly soluble and another internal donor compound is easily soluble, the amount of said slightly soluble donor compound in the catalyst system being at least 1 w-%. In other words, if transesterification method is used, said transesterification reaction is carried out only partly.
Said internal donor compounds are preferably brought in the procatalyst composition together with the titanium component. The titanation of the procatalyst composition is carried out at least twice. During the first titanation the molar ratio of the added phthalic acid ester and magnesium halide is preferably equal or greater than 0.1. During the second titanation the molar ratio of the added phthalic acid ester and magnesium halide is 0-0.3. If during the second titanation no phthalic acid ester is added, then further titanation steps are not necessary. However, if phthalic acid ester is added during the second titanation, the third or possibly more titanation steps are necessary.
The procatalyst composition is used together with an organometallic cocatalyst, like aluminium trialkyl, and preferably with an external donor, such like cyclohexyl methylmethoxy silane or dicyclo pentyldimethoxy silane.
The catalyst can also be prepolymerized prior to feeding into polymerization reactor. In the prepolymerization the catalyst components are contacted for a short period with a monomer before feeding to the reactor.
The transesterification method provides a convenient way to bring in the procatalyst composition at least one slightly soluble internal donor. However, any other methods can be used to bring in the procatalyst composition at least one internal donor compound, in an amount of at least 1 w-% is a slightly soluble internal donor compound.
The process described above makes it possible to produce polypropylenes having a molecular weight and melt index varying from low to very high and at the same time maintaining a high isotacticity index. A greater amount of polymer can be achieved by catalysts according to the invention compared to the traditional catalysts or greater production by volume can be achieved from the same reactor volume. The products have high elasticity or high crystallinity and high flexural modulus.
According to one embodiment of the invention the process comprises only one slurry reactor, which is operated at a temperature between 80xc2x0 C. and the critical temperature of the reaction mixture. This means that the temperature is generally between 80xc2x0 C. and 91xc2x0 C. The pressure has no upper limit, but for practical reasons the preferable pressures are in the range of 46-70 bar, preferably 50-70 bar.
The polymerization is carried out by feeding a catalyst system, a mixture of propylene acting as reaction diluent and optional hydrogen and comonomer into the slurry reactor. The polymerization heat is removed by cooling the reactor by cooling jacket. The residence time in the slurry reactor must be at least 15 minutes, preferably 20-100 min for obtaining a sufficient degree of polymerization. This is necessary to achieve polymer yields of over 40 kg PP/g cat.
According to one embodiment of the invention, light inert hydrocarbons are fed to the reactor. Examples of such hydrocarbons are iso-butane, n-butane and isopentane. The light, inert hydrocarbon in the polymerization mixture lowers the pressure required in the reactor. The increased catalyst activity at relatively high temperature compensates the decreased activity due to lowered concentration of propylene.
If lower molecular weight polypropylene is the desired product hydrogen can be fed into the reactor. Hydrogen can be added in the reactor 0.001-100 mol H2/kmol propylene, preferably in the range of 1.5-15 mol H2/kmol propylene.
Comonomers can be added into the reactor in any desired amount, preferably 0-20% of the monomer feed. Ethylene, butylene and hexene, among others, can be used as comonomers for the manufacture of polymers for blow molding sheets, pipe and film.
According to a preferable embodiment of the invention, it comprises two slurry reactors, which are operated at a temperature of 80-91xc2x0 C. The dual reactor system is used because it decreases the possibility that catalyst particles move unreacted to the second reactor. This would cause gels or difficulties in downstream because of high catalyst activity. The pressure can be between within the range of 35-70 bar, while preferably it can be less, eg. 40-60, if light hydrocarbons are added into the reaction mixture. Hydrogen can be present in the amount of 0-15 mol/kmol propylene feed, preferably 0-3 mol/kmol propylene. Because the polymerization temperature is high, the molecular weight distribution tend to be narrow, but can be controlled broad in two reactors by varying hydrogen concentration in different reactors. The residence time can be varied for example between 15-100 min such that the residence time in the second reactor can be the same or up to three times as that in the first reactor. This means that the reactor volume of the second reactor can likewise be the same or up to three times as that of the first reactor.
Hydrogen can be added in the second reactor at 0.001-100 mol H2/kmol propylene, preferably in the range of 1.5-15 mol H2/kmol propylene. The amount of hydrogen into the second reactor can be equal to or higher than that of the first reactor.
According to one preferable embodiment of the invention, two sequential loop reactors are used and the polymerization temperature in the first reactor is lower than in the second reactor. The polyme rization activity of the catalyst decreases in the first loop reactor, but this effect can be compensated in the second reactor due to higher temperature.
Comonomers can be added into the first reactor and second reactor in any desired amount, preferably 0-20% of the monomer feed. Ethylene, butylene and hexene, among others, can be used as comonomers for the manufacture of polymers for blow molding, sheets, pipe and film.
By this way propylene polymers having a broad or bimodal molecular weight distribution can be produced. The polymers have a high flexural modulus of 1700-2100 MPa.
If polymers having a broad or very broad or bimodal molecular weight distribution is desired, the slurry reactor or reactors can be followed by a gas phase reactor or reactors. By this way, higher comonomer contents can be used and multimodal products achieved. The polymerization in the gas phase can be carried out at a temperature of 60-100xc2x0 C. and in the pressure of 10-40 bar. It is desirable that no hydrogen or a minor amount of hydrogen is fed into the gas phase reactor. If hydrogen is applied, it is optionally removed from the reaction mixture before feeding the polymer into the gas phase reactor. This can be done by ordinary means, for example by cyclone separators or other suitable flash tank.
In this way high impact resistant products having a raised stiffness can be produced.